It is desirable to provide an effective process to increase the yield of gasoline (naphtha) in catalytic cracking units. More particularly, catalytic cracking of oil is an important resid hydrotreating unit process which is used to produce gasoline and other hydrocarbons. In order to produce more valuable, lower molecular weight hydrocarbons during catalytic cracking, a feedstock, which is generally a cut or fraction of crude oil, is cracked in a reactor under catalytic cracking temperatures and pressures while in the presence of a catalyst. Gas oil which is usually used as a feedstock in catalytic cracking typically contain from 55% to 80% gas oil by volume, having a boiling range from 650.degree. F. to 1000.degree. F. and less than 1% Ramscarbon by weight. Gas oil feedstocks usually contain less than 5% by volume naphtha and lighter hydrocarbons having a boiling temperature below 430.degree. F., from 10% to 30% by volume diesel and kerosene having a boiling range from 430.degree. F. to 650.degree. F., and less than 10% by volume resid having a boiling temperature above 1000.degree. F.
Known processes catalytically crack virgin unhydrotreated, low sulfur resid as well as deasphalt, subsequently hydrotreat, and catalytically crack high sulfur resid. Furthermore, such prior art processes produce hydrogen-rich asphaltenes which are difficult and expensive to handle, process, and melt (liquefy) at relatively low temperatures. These asphaltenes cannot be used as solid fuel, are difficult to blend into fuel oils, and are not generally usable and desirable for asphalt paving or for use in other products.
Refiners have used deasphalting processes to fractionate low sulfur reside ("LSR") and to enhance the processing of the resulting fractions. Typically, the low sulfur resid is fractionated into an oils fraction and a heavy fraction including resins and asphaltenes.
The oils fraction is a desirable feed for a catalytic cracking process because it contains relatively small amounts of metals, nitrogen, and refractory coke-forming compounds. Typical catalytic cracking yield from the oils fraction are similar to those obtained from virgin gas oils. The small amounts of metals and refractory compounds allow large amounts of the oils fraction to be processed in a catalytic cracker ("FCCU").
However, low sulphur resids are relatively poor feedstocks for a resid hydrotreating processes. In resid hydrotreating, the resid is subjected to a high temperature process in the presence of hydrogen and a hydrogenation catalyst. The objectives of these processes, such as a conventional hydrotreating process, is to remove sulfur, nitrogen, and metals, and to saturate olefins and aromatic compounds. In addition, the resid boils above 1000.degree. F. where it is converted to lighter products, which can be subsequently upgraded in other refining units. High sulfur reside ("HSR") is significantly more reactive than low sulfur resids in hydrotreating processes. These qualities limit the usefulness of the low sulfur resids in hydrotreating processes.
Compared to high sulfur resids, low sulfur ("LSR") and hydrotreated ("HTR") resids are desirable feeds for deasphalting because they give significant yields of oils, which in turn, are good feeds for catalytic cracking since they are low in metals, sulfur, and resins. However, it is not straightforward and easy to process LSR and HTR resids in a single deasphalter because it is desirable to segregate the asphalt fractions. Moreover, the HTR asphalt fractions is refractory, while the LSR resid is reactive, to hydrotreating and coking.
Therefore, it is desirable to provide means for and methods of deasphalting LSR and HTR in a single unit in order to segregate the components of HTR and LSR to best advantage.